This disclosure relates to phase-locked loops and more specifically, to a loop filter used in a phase-locked loop.
A phase-locked loop (PLL) is often used in designing a precise clock for a system. The PLL takes advantage of negative feedback to constantly adjust the frequency and phase of an oscillator that may change or drift. FIG. 1 is a simplified block diagram of the PLL. The PLL includes a voltage-controlled oscillator (VCO) 100, a phase and frequency detector 102, and a feedback frequency divider 104. The VCO 100 often takes a voltage 106 as its control input and outputs a signal 108 whose frequency is based on the value of the input voltage 106. The phase and frequency detector 102 operates in reverse. It takes two signals 109, 110 as its inputs and outputs a voltage 106 based on the difference between the frequencies of the two signals 109, 110.
A PLL in a computer system, for example, receives a reference frequency source 110, such as an external bus clock, and a feedback frequency 109 from the VCO as inputs to the phase and frequency detector 102. The feedback signal 109 frequency is the VCO output frequency divided by the feedback frequency divider 104. The output from the phase and frequency detector 102 is then used to control the VCO 100. When the PLL is locked, the frequency and phase of the reference signal 110 and of the feedback signal 109 are equal. The VCO output 108 frequency is N times the frequency of the reference signal 110 (N is the dividing ratio of the feedback frequency divider 104). If the VCO 100 starts to drift, the phase and frequency detector 102 detects and corrects the discrepancy.
The output of the PLL circuit can then be used to clock a processor, such as a central processing unit (CPU). Due to the feedback frequency divider 104, the CPU clock has a significantly higher frequency than the bus clock.
In a preferred design for the PLL, charge pumps and a loop filter are coupled between the frequency comparator 102 and the VCO 100 to control the VCO output frequency. The charge pumps feed pulses of current to a capacitor in the loop filter. The current pulse charges and discharges the loop filter capacitor.
A loop filter in the phase-locked loop includes a capacitor having a first capacitance. The loop filter also includes an amplifier coupled to a node of the capacitor. The amplifier amplifies a signal at the node in a way that increases the first capacitance without physically changing the capacitor.